In recent years, astronomers have found that they can see both the emissions from the supermassive black hole as well as the emissions from the gas clouds. These are obviously related but the time it takes for radiation to reach the cloud means that changes here lag those in the supermassive black hole.

This delay, which can be measured with a technique called reverberation mapping, is then clear measure of the radius of the cloud.

But since the flux of the radiation from the black hole drops as an inverse square law, the brightness of these clouds also depends on their radius.

So a good measure of their radius also gives an indication of their intrinsic brightness.

Now Watson and co have used this technique to measure the distance to 38 active galactic nuclei at distances of up to z=4. That's significantly further than is possible with type 1a supernova, whose distance cannot be accurately measured beyond z=1.7.